1. Field of the Invention
The present invention relates to a wet-type compacting method for powder, a production method for a sintered powder compact obtained by sintering a compact obtained by the wet-type compacting method of powder, a sintered compact obtained by the production method, and an application device thereof.
2. Description of the Prior Art
When producing a product by compacting and sintering a powder such as ceramics or powder metallurgical materials, it is known that production of a large and complicated shaped product is extremely difficult.
First of all, it is difficult to compact the powder. Since strength of a large and complicated shaped powder compact is low, the compact is brittle and easily damaged, and its handling is difficult. And a large and complicated shaped powder compact cannot be obtained by only pressing the powder. In this case a technique of wet-type compacting is usually adopted whereby a mixture of the powder and a solvent is produced anywhere in a compacting process and a compact or the precursor thereof is formed from the mixture. The compact or the precursor thereof made from the mixture of the powder and the solvent is often applied with external force during processing. To avoid unexpected damage, the mixture of the powder and the solvent is required to provide plasticity. However, an ordinary mixture of the solvent and the powder is seldom provided with plasticity unless it contains a large amount of plastic powder such as clay.
To relieve such difficulty in the compacting process, a compacting aid may be added to the mixture of powder and solvent. Such a compacting aid includes, for example, a binder for imparting strength to the dried compact, or a plasticizer for imparting plasticity to the mixture of powder and solvent.
High polymer organic substance is mainly used as a compacting aid such as the binder or plasticizer, but it is desirable that these compacting aids are uniformly dispersed in the mixture of powder and solvent. To create a condition where the high polymer organic substance is uniformly dispersed in the mixture of powder and solvent, according to a conventional technique, a high polymer organic substance which is soluble in the solvent or is dispersible in a form such as an emulsion is used, and such a technique is disclosed, for example, in Japanese Unexamined Patent Publication No. HEI 7-267741, Japanese Unexamined Patent Publication No. HEI 10-1366 or the like.
To coat the surface of the powder with an organic high polymer in advance to compact the powder, a technique is known whereby, when silicon nitride powder of which the water resistance is low is compacted with a solvent of a water system, the powder is coated with a resin which is insoluble in water to impart water resistance to the powder. This technique is disclosed in Japanese Unexamined Patent Publication No. HEI 6-100372. However, this does not impart plasticity and strength to the compact, therefore it is necessary to separately add a compacting aid such as a water-soluble plasticizer or a binder. Japanese Unexamined Patent Publication No. 2000-264696 also discloses a technique whereby the surface of a powder is coated with resin and then compacted using an organic solvent. However, this resin is soluble in the organic solvent. Plasticity is thus imparted by making use of solubility.
The compacting process was described above in detail, but another reason for difficulty in producing a large and complicated shaped product is that uniform and dense sintering is so difficult.
To sinter a powder compact uniformly and densely, it is necessary to uniformly and compactly pack the powder for the compact. In addition, a method of adding a sintering aid for promoting sintering to a mixture of powder and solvent or sintering by hot pressing under conditions of high temperature and high pressure can be applied. It is also known that the more minute the ceramic powder, the easier the sintering. However, it is to be noted that the more minute the powder, the more difficult the compacting.
Description was made above as to how difficult is the production of a large and complicated shaped product in the compacting and sintering process, and carbide ceramics such as silicon carbide or boron carbide are considered to be difficult materials to compact and sinter. Among these, boron carbide ceramics are considered to be more difficult in processing. Boron carbide sintered compact is usually produced by hot pressing because of its difficulty in sintering. Japanese Unexamined Patent Publication No. HEI 7-97264 discloses a production method by atmospheric pressure sintering. However, it is impossible to produce products of a large and complicated shape in this method and the maximum relative density of sintered compact is about 96%.
Slip casting is most suitable for producing large and complicated shaped products among various compacting techniques, but it is the most technically difficult compacting method. Slip casting techniques for silicon carbide among the carbide ceramics that are difficult to compact are disclosed in Japanese Unexamined Patent Publication No. HEI 6-144915 and the like. However, for boron carbide that is more difficult to compact, a slip casting technique is not yet known.
An application field for a sintered powder compact, in particular, a ceramic sintered powder compact includes a movable section for a mobile body device having a highly accurate positioning function. The mobile body device which requires the highly accurate positioning function includes a three-coordinate measuring machine, a straightness measuring device, and a lithography machine for forming patterns on a plain object or the like. In the mobile body device provided with such a highly accurate positioning function, a small type of device provided with such a bearing as disclosed in Japanese Unexamined Patent Publication No. HEI 5-174520 is known. As a large type of device, a hydrostatic fluid bearing device is mainly used. In particular, in the lithography machine, when a modern semiconductor wafer or a liquid crystal panel is produced, a highly accurate positioning function corresponding to the finer microstructure of the patterns is required. At the same time, to form the patterns economically, it is necessary that the mobile body loaded with a workpiece such as a semiconductor wafer or a liquid crystal panel to be exposed and a reticle or the like, is moved at high speed to improve the throughput of the device. However, to move the mobile body at high speed inevitably generates vibration that is a minus factor with reference to positioning accuracy. Further, to move the mobile body at high speed under a fixed driving force, it is necessary to lighten the movable section.
To cope with both the high speed and the high positioning accuracy, it is necessary to use a mobile body constructed from a material with a large specific rigidity ratio (Young's modulus/Specific gravity). Accordingly, in place of a conventional material of a metal system, the mobile body device adopting the movable section using a construction material made of ceramics has become available in recent years. For example, Japanese Unexamined Patent Publication No. HEI 4-347008 discloses that a fluid bearing made of ceramics is superior to a metal bearing in specific rigidity. A fluid bearing made of alumina is disclosed in Japanese Unexamined Patent Publication No. HEI 6-297421 as an embodiment thereof. Japanese Unexamined Patent Publication No. HEI 6-297421 also discloses examples of ceramic materials used in the fluid bearing, wherein silicon nitride and silicon carbide are mentioned, which will provide a ceramic sintered compact with a larger specific rigidity ratio than Alumina if they are completely densely sintered. Japanese Unexamined Patent Publication No. 2000-182945 also discloses members for a lithography device in which silicon carbide and boron carbide having a large specific rigidity ratio are used.
An application field for the sintered powder compact, in particular, ceramic sintered powder compact includes a protective member for efficiently absorbing shock from collision with a missile.
Development of a protective member for a human body, structure, mobile body or the like to absorb shock from collision with a missile has been an important theme since the beginning of the history of man, and continuous progress has been made from ancient shields and armatures for protecting from arrows and javelins to present protective structures for a spacecraft in collision with meteorites. Metal has consistently played a leading part in such development, in particular, alloys of iron. Even if the heat resistance of iron has come under question since entering the space age, metal is still regarded as important material thanks to the development of heat-resistant special steels and the like.
Superiority of metal as the protective member can be found in two points: impact strength and workability. One more characteristic essential for the protective member is extra hardness. Recommended as an industrial material for extra hardness is a ceramic sintered powder compact. However, ceramic material is generally regarded as solid, but brittle and easily cracked. It is true that the ceramic material is inferior to metal in impact strength and there has been no trials for using ceramic material as the protective member for absorbing the shock of collision with a missile up to recent years.
However, a protective member using a ceramic material called Chobam composite armor has been recently developed. Ceramics cannot overcome their own brittleness alone, but a new structure which can absorb shock was attained by combining a ceramic plate with an extremely tough steel plate or by partly adopting a hollow structure therein. The background of appearance of such a structure is in that the main object to be protected against has changed from a missile such as AP, APC, APDS, and APFSDS mainly utilizing kinetic energy whereby the impact is limited by the rate of fire to a missile such as HEAT mainly utilizing chemical energy by jetting in the vicinity of the protective member, and it has become necessary to protect from shock caused by collision with the missile of which the speed has become extraordinarily high compared with a conventional one. This necessity increases more when the protective member is used in a spacecraft or the like which needs protection from a missile which flies faster.
Since protective members using ceramics were put to practical use, attention has been paid to its lightness compared with metal, and, in particular, development has advanced mainly for use in the aerospace industry. In a protective member aiming at lightening, a structure of ceramic tile provided with a backup layer of fiber reinforced plastic is common. The structural examples are disclosed, for example, in U.S. Pat. Nos. 4,739,690, 5,996,113, WO 98/51988 and the like.
However, there have been the following problems in the prior art.
When a compacting aid of organic high polymer is used in the wet-type powder compacting process and, for example, slip casting is adopted as a compacting method, the compacting aid soluble in a solvent often causes clogging in a mold. And the compacting aid soluble in the solvent sometimes causes its segregation in a compact in the process of drying the solvent from the compact and as a result, the compact after drying becomes heterogeneous. The heterogeneous compact remains heterogeneous even in the next sintering process and, as a result, a heterogeneous sintered compact is only available. And in compacting a non-plastic body, even if the compacting aid imparting plasticity is added according to a conventional method, the plasticity is still inferior compared with plastic body such as a clay base body. Accordingly, there is a limit to the size and shape of the compact.
Since these compacting aids must be burnt off in the next sintering process, the density of a sintered body decreases and as a result, satisfactory physical properties of the sintered body cannot be obtained. It is also difficult to uniformly disperse an ordinary sintering aid in a mixture of powder and solvent. As a result, the sintering aid heterogeneously existing in the compact results in heterogeneous physical properties of the sintered compact. Further, according to a method of promoting sintering by hot pressing under high temperature and high pressure, increase of manufacturing cost is inevitable. Basically, it is not possible to manufacture a large and complicated shaped product from the limits of the equipment by hot pressing.
Accordingly, with reference to carbide ceramics which are difficult to compact and sinter, and in particular, to boron carbide, it has been regarded as completely impossible to manufacture a large and complicated shaped product at low cost until now.
Further, when a sintered powder compact, in particular, a ceramic sintered powder compact is applied to a movable section of a mobile body device having a highly accurate positioning function, if, for example, alumina is used, the specific rigidity ratio of alumina is about 80˜95 GPa even if the alumina is sintered until its Young's modulus reaches close to the maximum value. The specific rigidity ratio in such a case is not enough to attain the superb throughput and positioning accuracy which are required, for example, in a lithography machine.
Ceramics such as silicon nitride, silicon carbide or boron carbide are materials difficult to sinter and compact compared with alumina and it is difficult to economically compact and sinter a large product which can be used in the movable section of the lithography machine. In particular, in the conventional manufacturing method, hot pressing is required to manufacture a large product, not a small test piece, which can be put to practical use. However, as far as hot pressing is used, the manufacturing cost is enormous and the complicated shaped product cannot be produced.
Further, to improve the specific rigidity ratio of the movable section in the lithography machine, not only material of high specific rigidity ratio but also a rib structure or hollow structure should be adopted to improve the apparent specific rigidity ratio. However, when ceramic material of high specific rigidity ratio is used, adoption of such a structure results in more difficulty in compacting and sintering. Accordingly, it has been considered that adoption of the rib structure or hollow structure was impossible until today.
Still further, when a sintered powder compact, in particular, a ceramic sintered powder compact is applied to a protective member for efficiently absorbing the shock from collision with a missile (a flying object), in other words, when the ceramic material is used as a component of the protective member for absorbing the shock from collision with the missile, it is not possible to produce a large and complicated shaped product. In this case, a plate or tile shaped ceramic members can only be used in combination. Accordingly, when such ceramic members are applied to the protective member of the complicated shaped device, there is a limit to combination of the plate-shaped ceramic members and the joint section causes a problem. For example, U.S. Pat. Nos. 4,739,690, 5,996,113, WO 98/51988 and the like disclose such a joint structure, wherein techniques whereby a joint section is thickened to provide build-up, a butt joint structure is provided, or an adhesive agent is used, are disclosed. However, in such a structure, fragility of the joint section cannot be sufficiently covered.
It is also difficult for the protective member of a tabular structure to be arranged obliquely to the predicted direction of collision with the missile. A conventional protective member of metal material has a curved structure in many cases, but a ceramic composite protective member can only have a square structure which has a drawback of inferior defending ability.
And it is considered that a ceramics composite armor of a sandwich structure should adopt a rib structure or hollow structure to disperse the shock, but a combination of plate-shaped ceramics cannot have such a structure.